Explore the concept of function composition in building Rack applications through this RubyConf 2021 talk by Marc Busqué. Delve into the fundamentals of functions as black boxes that process inputs and produce outputs, drawing parallels with HTTP requests and responses. Learn how web_pipe facilitates the creation of modular Rack applications by connecting small processing units. Discover the power of function composition, understand hidden preconditions and side effects, and gain insights into Rack application development. The talk covers key topics including Webpipe, types, steps, modularity, parameters, and touches on Hanami framework integration.